Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58..471l&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 1, pp.471-494
Astronomy and Astrophysics
Astrophysics
98
Scientific paper
We have analyzed He, Ne, Ar, Kr, and Xe in fourteen size fractions of interstellar SiC, isolated from the Murchison C2 chondrite. All are mixtures of a highly anomalous component bearing the isotopic signature of the astrophysical s-process and a more normal component, generally solar-like but with anomalies of up to 30% in the heavy isotopes. As these two components strikingly resemble predictions for the He-burning shells and envelopes of red giant carbon stars, it appears that the SiC grains are pristine circumstellar condensates from such stars. A number of elemental and isotopic ratios (such as K 80 / K 82 and K 86 / Kr 82 ) vary with grain size, suggesting that the SiC comes from carbon stars representing a range of masses, metallicities, temperatures, and neutron densities. The Ne 21 -content of the SiC suggests a presolar cosmic-ray irradiation of up to 130 Ma, representing the interval between formation of the grains in a circumstellar shell and arrival in the solar system 4.6 Ga ago. Actually there is evidence that most of the Ne 21 (and Ne 22 ) is in 10% of the grains, suggesting that much of the SiC was degassed during or shortly before formation of the solar system. Thus the true cosmic-ray ages may be 7 to 18 × longer. Apparently the gas-rich SiC grains predate the solar system by at least 130 Ma and possibly up to 2000 Ma.
Amari Sachiko
Anders Edward
Lewis Roy S.
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